Experimental relationships between levels of corticosterone in plasma and feathers in a free-living bird

Integrated measures of corticosterone (CORT), such as from feathers (CORT_f), have intuitive appeal because they incorporate both the duration and amplitude of glucocorticoid secretion. An association between CORT_f and plasma CORT has never been shown in wild birds, and it is unclear as to when and if these measures should be correlated, given that they are fundamentally different yet related measures of physiology. We hypothesized that CORT_f should correlate with instantaneous measurements of plasma CORT when the latter reflect sustained changes in the activity of the hypothalamic-pituitary-adrenal (HPA) axis. To test this, we experimentally manipulated levels of plasma CORT in wild nestling tree swallows (Tachycineta bicolor) using 5-day time-release CORT pellets, and measured plasma CORT and growth parameters before, during, and at the end of hormone manipulation (days 7, 9, and 11 post-hatch, respectively). CORT_f and plasma CORT were significantly positively related only when the latter was at its highest and most variable among individuals (day 9). A similar relationship was expected at day 11, but plasma CORT had returned to near original levels. Nestlings with higher CORT_f were smaller, lighter, and less likely to fledge, but we did not detect seasonal effects on CORT_f. Our results clearly demonstrate that CORT_f from free-living birds can reflect plasma CORT, but correlations may not always be expected, especially if elevations in plasma CORT are relatively modest and of short duration. Our work suggests that CORT_f is best used to study activity of the HPA axis over relatively long time frames and can be used effectively to advance avian ecology.